Backer for electron beam hole drilling

Abstract

Disclosed is an electron beam hole drilling process in which the workpiece surface where the hole exits is covered with a backer comprising particulate bonded to itself and the exit surface by a binding agent. The backer enables a hole having substantial uniform symmetry along its length to be drilled and is particularly useful for hollow parts and contoured surfaces. Backers removable by liquifying, melting dissolution, leaching and the like, are particularly useful for inaccessible exit surfaces. In a preferred embodiment the backer is comprised of a nonmetallic particulate such as glass while the binder is a soluble silicate.

Having thus described a typical embodiment of our invention, that which we claim as new and desire to secure by Letters Patent of the United States is: 
     
       1. A method for drilling a hole with an energy beam, such as an electron beam, through a workpiece having an entrance surface toward which the beam propagates and through which it first penetrates, and an exit surface which the beam last penetrates, comprising: (a) intimately contacting a backer with the exit surface, the backer being characterized by a particulate bonded together with a binder, the particulate and binder having properties which avoid forming reaction products deleterious to the workpiece during hole drilling and the backer being further characterized by being adapted to removal, at least in part, as a liquid after drilling;   (b) directing an energy beam onto a portion of the entrance surface with an intensity sufficient to produce a hole in the workpiece, penetrate the exit surface, and generate within the backer gaseous products sufficient to expel molten workpiece material from the hole at the entrance surface in a direction opposite to the beam propagation; and   (c) removing the backer by converting it to at least a partial liquid state with a means wich does not adversely affect the workpiece.   
     
     
       2. The method of claim 1 comprising the additional step of applying the backer to the workpiece as an adherent coating. 
     
     
       3. The methods of claims 1 or 2 where the backer is further characterized by being meltable and the removal means is heating. 
     
     
       4. The methods of claims 1 or 2 where the backer is further characterized by being leachable from the workpiece surface as a liquid. 
     
     
       5. The method of claims 1 or 2 where the particulate is an inorganic nonmetallic and the binder is a polymer liquifiable at a temperature less than about 300° F. (150° C.) 
     
     
       6. The method for drilling a hole with an electron beam in a hollow gas turbine airfoil article having a beam entrance surface defining the exterior of the article, toward which the beam propagates and through which it first penetrates, and an interior surface defining a cavity, into which the beam last penetrates, comprising: filling the interior cavity with a backer, the backer being characterized by an inorganic nonmetallic particulate bonded to itself and to the interior surface with a polymeric binder, the materials and quantities being chosen to prevent beam penetration and contain the gas pressure generated therewithin by the action of an energy beam;   directing an energy beam onto a portion of the entrance surface of the article with an intensity sufficient to produce a hole in the wall of the article and to additionally penetrate the backer;   generating sufficient vapor pressure under the action of the beam on the backer to expel molten workpiece material from the hole at the entrance surface, in a direction opposite to the beam propagation;   preventing the beam from striking the wall opposing the wall being drilled, by absorption of excess energy in the backer; and   removing the backer by converting it at least in part to a liquid, using a means which does not adversely affect the article, such means being chosen from either melting or leaching; to form a hole of substantial uniform symmetry along its length in the article wall and to minimize the formation of burrs, recast layers, and other hole defects which lower the performance of the article.     
     
     
       7. An article comprising: a metal workpiece having an entrance surface for receiving impingement of beam energy capable of drilling a hole therein, and an exit surface;   a backer placed in a gas tight contact with the exit surface of the workpiece, the backer consisting of: (a) particulates adapted to absorb beam energy and thereby become vaporized when impinged upon, and   (b) a binder, intermixed with and intimately surrounding the particulates for adhering them to each other, the binder being vaporizable under action of the beam and being convertible to a liquid after drilling without deleterious effect on the workpiece;     the backer being adapted to removal from the workpiece after drilling by conversion of the binder to a liquid; and   the backer having sufficient combination of particulate, binder, and thickness to provide for beam absorption and for generation of sufficient gas to expel molten metal from a hole at the entrance surface.   
     
     
       8. The article of claim 7 wherein the particulates are nonmetallic powders, such as ceramics or glasses, and wherein the binder is an organic polymer convertible into a liquid at a temperature less than about 300° F. (150° C.). 
     
     
       9. The article of claim 7 wherein the backer is provided as a coating adhered to the exit surface by the binder. 
     
     
       10. The article of claims 7, 8 or 9 wherein the particulates are greater than 5 microns average particle size and comprise 50-90 percent by weight of the backer, and wherein the backer thickness is about 1.5 to 6 mm (0.06 to 0.25 inch). 
     
     
       11. The method of drilling a hole with an energy beam, such as an electron beam, through the first section of a workpiece having at least two spaced apart sections lying along the path of the beam, wherein the space between the sections is at least partially filled with a solid backer in intimate contact with the beam exit surface of the first section during hole dirlling, and wherein the backer is converted into at least a partial liquid state while it is being placed into and removed from the space. 
     
     
       12. The method of claim 11 wherein the workpiece is a tubular article and the first section is a wall of the article and the second section is an opposing wall.